The present invention relates to an amphibious vehicle.
Amphibious vehicles are inherently required to carry both road vehicle and marine vessel dedicated equipment. Hence, amphibious vehicles are generally heavier than boats of a similar size and power. This is particularly true in the case of planing amphibians, which require a relatively heavy wheel streamlining mechanism, such as retractable suspension or the like. In the case of an amphibious vehicle with open wheel arches, the drag exerted by the arches can be considerable and hence the overall hull drag is greater than for a conventional boat. This additional weight and drag make it difficult for the vehicle to rise onto the plane without initial deployment of trim tabs.
Trim tabs are well known for marine vessels, and in particular planing boats, as means for controlling the trim and attitude of the vessel to compensate for changes in load, speed or sea conditions. Commonly these devices are in the form of flat tabs or planes, which are pivotably connected along a generally horizontal axis to the hull of the vessel below the waterline near the stem. The angle of orientation of the trim tabs is adjustable and hence determines the fore and aft attitude of the vessel, when it is being propelled through the water. For example, if the rear of the vessel is heavily loaded the bow will typically rise out of the water and lead to inefficient forward motion. In these circumstances the angle of the trim tabs relative to the hull can be increased to lower the bow, lift the stem, and hence get the boat back onto a plane.
The position of the trim tabs on a conventional boat, when docked, is not of great importance since it is unlikely that the tabs will come into contact with, and be damaged by a submerged obstacle. Nevertheless, since trim tabs are typically extended by means of hydraulic actuators it is generally desirable for trim tabs to be retracted, when the boat is not in use, to avoid unnecessary marine growth on the actuators. Retraction also minimises the risk of damage when a boat is lifted from the water, for example, to go into dry storage. In accordance with this U.S. Pat. No. 5,113,780 discloses a trim tab control system, which includes a facility for automatically retracting the trim tabs when the engine ignition is switched off.
The system described in U.S. Pat. No. 5,113,780 is designed specifically for marine only vessels and is of limited benefit in the case of amphibious vehicles fitted with trim tabs. The greatest risk of damage, in the case of amphibious vehicles, occurs during and after the transition from marine to terrestrial mode, whilst the engine is still switched on. If the tabs remain in an operational position when the vehicle leaves the water, there is a significant risk that they may strike the ground at speed. Similarly, reversing into an obstacle with the trim tabs deployed could damage the tabs, the actuators and the body of the vehicle. The risk of inadvertently leaving the trim tabs deployed is enhanced because during the marine to terrestrial transition period the operator has several other important control tasks to consider.
Furthermore, the requirement in U.S. Pat. No. 5,113,780 that the tabs be retracted after the ignition is switched off necessitates an ignition independent power supply for powering the retraction system. Such a supply, however, is undesirable because the power source remains permanently connected to the circuit that it powers. Hence, even a relatively small current drain from the retraction circuit could lead to a significant reduction in the life of the source. Whilst a permanent current drain may be unlikely to arise as a result of poor design there is a significant probability that it could occur as a result of malfunction. The risk of this is particularly high in marine vessels where the presence of water makes short circuits more likely. It will also be appreciated that short circuits of this type also constitute a safety hazard.
This latter problem has been tackled in U.S. Pat. No. 5,474,013, which describes a system for automatically retracting trim tabs on a boat independently of the operation of the ignition switch. In the system described, a capacitor is charged from the vessels battery and the charge stored can be discharged to fully retract the trim tabs. However, this system is also designed specifically for marine only vessels and again is of limited benefit in the case of amphibious vehicles, for the reasons discussed above in relation to U.S. Pat. No. 5,113,780.
There is, therefore, a need for an amphibious vehicle, which includes a trim tab deployment system to allow it to rise onto the plane. There is also a need for the trim tab system for the amphibious vehicle to overcome or at least to mitigate the problems referred to above.